Seeing Cost-Efficiency in a New Light: The Future of Solar Cells

It is a commonly cited fact that the Earth receives more energy from the sun in an hour than the whole planet uses in a year. That’s an impressive statistic, but it’s also a frustrating one—the solution to our global energy crisis appears to be staring us in face yet we can only harness a small fraction of it. Here, I want to take a look at why solar energy is so promising, explore emerging solar technologies, and try to understand what the holdup is.

It's not always obvious that almost all of the energy we use comes from the light of the sun with the exceptions of nuclear and geothermal power. Solar heating creates the wind that turns turbines and drives the water cycle that keeps rivers flowing through hydroelectric dams. Fossil fuels are just the old, compressed remains of ancient organisms that got their energy fix from photosynthesis.

Thermodynamics tells us that when energy is converted from one form to another, some of it is lost. So, the more times you convert energy, the lower your efficiency becomes. Let’s reconsider fossil fuels and count the number of times energy is converted. Plants convert sunlight into organic molecules which are then burned to create heat to boil water, generating steam that will turn a turbine generating electricity. Compare this with solar technology, which directly converts sunlight into electricity, and it’s not hard to understand the immediate advantages of solar.

The classic, shiny blue solar panels that adorn rooftops represent the oldest solar technology we have—the so-called first generation cells. These cells are made of two types of materials pressed together. One is the P-type material (P for positive), which is engineered so that it is missing electrons and wants them back. The other material is N-type (N for negative) and has extra electrons. The electrons want to flow from where there are extra to where there aren’t enough—they’re very generous that way—and therefore move from N-type to P-type.

After a while, this electron flow will come to equilibrium and no more electrons will be moving around. This equilibrium creates a driving force in the solar cell called an electric field. If you inserted an electron into the cell with this electric field, it would get pushed toward one side. This is exactly what light does when it strikes the solar cell—it frees an electron which then gets pushed. Pushing electrons through a device is just another way of saying generating electricity.

We’ve gotten pretty good at making solar cells that work like this. The materials we use are usually crystalline silicon spiced up with other elements like phosphorous and boron. The main problem here is that purifying, crystalizing, and preparing sheets of silicon is still very energy intensive, and though the energy output of these cells outweighs the inputs in the long run, they still have high upfront costs.

To solve this problem, scientists and engineers have developed many new solar cell designs known as second and third generation or 'emerging' solar technology. There is a class of solar cells called thin-film solar cells which use light-harvesting materials deposited in thin layers on sheets of glass or plastic. These have several distinct advantages over crystalline silicon cells, namely that they are lighter, more flexible, and cheaper. The main drawback of these models is that they are not as efficient. Currently, the most efficient solar cells we have are called multijunction cells, with a power conversion of about 45 percent. Multijunction just means that instead of combining one P-type material with one N-type to make the cell, you combine many layers of these materials to create many junctions instead of just one. You can imagine that if creating a single P-N junction is expensive, it must be pretty pricey to make several.

If efficiency is so expensive, maybe it isn't what we should try to maximize. But emerging solar technologies are aiming to balance efficiency with cost: There are dozens of new types of cells that can be made cheaply and can deliver between 10 and 20 percent efficiencies. Their designs range from using organic polymers instead of silicon to using light-harvesting biological molecules like chlorophyll to excite electrons. The Massachusetts Institute of Technology recently announced the creation of a cheap, transparent material that could make windows into solar panels.

Even if these relatively low efficiency cells can’t power the world, they may help make the transition to fossil-free energy easier. It seems to me that if we can find a way to balance cost with efficiency, we will be on our way to a bright and solar-powered future.